1. Field of the Invention
The present invention relates to an ink jet recording apparatus for stably performing recording by ejecting an ink from a recording head to a recording medium and also to a temperature calculation method for calculating a temperature drift of the recording head.
2. Related Background Art
In the recent industrial fields, various products for converting input energy into heat, and utilizing the converted heat energy have been developed. In most of such products utilizing the heat energy, the relationship between the time and the temperature of an object obtained based on the input energy is an important control item.
A recording apparatus such as a printer, a copying machine, a facsimile machine, or the like records an image consisting of dot patterns on a recording medium such as a paper sheet, a plastic thin film, or the like on the basis of image information. The recording apparatuses can be classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, and the like. Of these types, the ink jet type apparatus (ink jet recording apparatus) ejects flying ink (recording liquid) droplets from ejection orifices of a recording head, and attaches the ink droplets to a recording medium, thus attaining recording.
In recent years, a large number of recording apparatuses are used, and have requirements for high-speed recording, high resolution, high image quality, low noise, and the like. As a recording apparatus which can meet such requirements, the ink jet recording apparatus is known. In the ink jet recording apparatus for performing recording by ejecting an ink from a recording head, stabilization of ink ejection and stabilization of an ink ejection quantity required for meeting the requirements are considerably influenced by the temperature of the ink in an ejection unit. More specifically, when the temperature of the ink is too low, the viscosity of the ink is abnormally decreased, and the ink cannot be ejected with normal ejection energy. On the contrary, when the temperature is too high, the ejection quantity is increased, and the ink overflows on a recording sheet, resulting in degradation of image quality.
For this reason, in the conventional ink jet recording apparatus, a temperature sensor is arranged on a recording head unit, and a method of controlling the temperature of the ink in the ejection unit on the basis of the detection temperature of the recording head to fall within a desired range, or a method of controlling ejection recovery processing is employed. As the temperature control heater, a heater member joined to the recording head unit, or ejection heaters themselves in an ink jet recording apparatus for performing recording by forming flying ink droplets by utilizing heat energy, i.e., in an apparatus for ejecting ink droplets by growing bubbles by film boiling of the ink, are often used. When the ejection heaters are used, they must be energized or powered on as not to produce bubbles.
In a recording apparatus for obtaining ejection ink droplets by forming bubbles in a solid state ink or liquid ink using heat energy, the ejection characteristics vary depending on the temperature of the recording head. Therefore, it is particularly important to control the temperature of the ink in the ejection unit and the temperature of the recording head, which considerably influences the temperature of the ink.
However, it is very difficult to measure the ink temperature in the ejection unit, which considerably influences the ejection characteristics as the important factor upon temperature control of the recording head, since the detection temperature of the sensor drifts beyond the temperature drift of the ink necessary in control because the ejection unit is also a heat source, and since the ink itself moves. For this reason, even if the temperature sensor is merely arranged near the recording head to measure the temperature of the ink upon ejection with high precision, it is rather difficult to measure the temperature drift of the ink itself.
As one means for controlling the temperature of the ink, an ink jet recording apparatus for indirectly realizing stabilization of the ink temperature by stabilizing the temperature of the recording head is proposed. U.S. Pat. No. 4,910,528 discloses an ink jet printer, which has a means for stabilizing the temperature of the recording head upon recording according to the predicted successive driving amount of ejection heaters with reference to the detection temperature of the temperature sensor arranged very close to the ejection heaters. More specifically, a heating means of the recording head, an energization means to the ejection heaters, a carriage drive control means for maintaining the temperature of the recording head below a predetermined value, a carriage scan delay means, a carriage scan speed decreasing means, a change means for a recording sequence of ink droplet ejection from the recording head, and the like are controlled according to the predicted temperature, thereby stabilizing the temperature of the recording head.
However, the ink jet printer disclosed in U.S. Pat. No. 4,910,528 may pose a problem such as a decrease in recording speed since it has priority to stabilization of the temperature of the recording head.
On the other hand, since a temperature detection member for the recording head, which is important upon temperature control of the recording head, normally suffers from variations, the detection temperatures often vary in units of recording heads. Thus, a method of calibrating or adjusting the temperature detection member of the recording head before delivery of the recording apparatus, or a method of providing a correction value of the temperature detection member to the recording head itself, and automatically correcting the detection temperature when the head is attached to the recording apparatus main body, is employed.
However, in the method of calibrating or adjusting the temperature detection member before delivery of the recording apparatus, when the recording head must be exchanged, or contrarily, when an electrical circuit board of the main body must be exchanged, the temperature detection member must be re-calibrated or re-adjusted, and jigs for re-calibration or re-adjustment must be prepared. In order to provide the correction value to the recording head itself, the correction value must be measured in units of recording heads, and a special memory means must be provided to the recording head. In addition, the main body must have a detection means for reading the correction value, resulting in demerits in terms of cost and the arrangement of the apparatus.
In the method of using the ejection heaters in temperature control, two major methods are proposed. One method is a method of simply using the ejection heaters in the same manner as a temperature keeping heater. In this method, short pulses, which do not cause production of bubbles, are continuously applied to the ejection heaters in a non-print state, e.g., in a standby state wherein no recording operation is performed, thereby keeping the temperature. The other method is a method based on multi-pulse PWM (pulse width modulation) control. In this method, in place of keeping the temperature in the non-print state such as the standby state, two pulses per ejection are applied to each heater, so that the temperature of the ink at a boundary portion with the heater is increased by the first pulse, and a bubble is produced by the next pulse, thus performing ejection. In order to change the ejection quantity in this method, the pulse width of the first pulse which is ON first is varied within a bubble non-production range to increase the energy quantity to be input to the heater, thereby increasing the temperature of the ink located at an interface portion with the heater.
However, the above-mentioned method, which is executed for the purpose of stabilizing the ejection quantity, has the following problems to be solved.
In the method using the temperature keeping heater, the entire head having a large heat capacity must be kept at a predetermined temperature by the temperature keeping heater, and extra energy therefor must be input. In addition, the temperature rise requires much time, and results in wait time in the first print operation. Furthermore, in a portable recording apparatus, since a battery must also be used for keeping the temperature, the maximum print count is undesirably decreased. When the temperature keeping heater and ejection heaters are simultaneously turned on, a large current must instantaneously flow through a power supply, a flexible cable, and the like, thus increasing cost and disturbing a compact structure.
In the method using the multi-pulse PWM control, since the pulse width of the second pulse for bubble production is fixed, and that of the first pulse is varied to vary the energy quantity to be input to the head so as to vary the ejection quantity, energy larger than normal must be supplied to the head in order to obtain the maximum ejection quantity. Therefore, although real-time characteristics can be remarkably improved as compared to the method using the temperature keeping heater, a further improvement is required for instantaneous power and the load on the battery.
It is also required to record a halftone image by controlling the ink ejection quantity according to a halftone signal. However, in the above-mentioned ejection quantity control, the ejection quantity variation range is not sufficient, and is required to be further widened.